This invention is directed to ejection devices and methods for stripping or demolding articles from multistation molding machines. More particularly, this invention teaches an innovative linking mechanism, between the ejection means of each molding station of a stack mold, which is independent relative to the motive means and linkage mechanism used to open/close the mold stations of a stack mold.
Generally speaking, a molded article may be ejected from a mold using mechanical means, such as stripper plates or stripping pins, or non-mechanical means, such as pressurized air. The mechanical ejection means are actuated using various motive means that are either part of the mold or part of the molding machine. All these ejection and motive means have been successfully used in conjunction with a single face (one station) type of mold.
Ejection of molded articles from a multistation mold can be achieved using the same means as mentioned for single face molds. Unfortunately, this approach does not represent an optimum solution taking into account the number of components that have to be used to serve each molding station. Based on the injection sequence within each molding station of a multistation mold, multistation molds can be used for either sequential or simultaneous injection processes. In a sequential process, at a certain time one station is used for a different molding step than the other(s). For example, in station A, one article may be in the cavity undergoing a cooling process, while in station B an article can be either ejected (if it has been in a cooling step prior this step) or an article can be injected in an empty cavity (if an article has been ejected before from this station). Accordingly, in a sequential process, a molding station can be in the open position (for ejection) while another molding station is in a closed position (for injection or cooling). This type of mold is sometimes known as a tandem mold. A tandem mold is useful when injecting large articles that have to remain in the closed mold to be cooled before being ejected. It thus appears that the only way to eject articles during a sequential process carried on using a multistation mold is by using any ejection means suitable for one face molds.
In most molding applications where the plastic articles are relatively small and do not require a long cooling time in the mold, a simultaneous injection process is carried on using a so called stack mold. In a stack mold, articles are injected, cooled and ejected simultaneously in a plurality of molding stations that are located between the fixed and the movable platen of a molding machine. Therefore, in a stack mold all stations are either open or closed simultaneously. Ejection of the molded articles from a stack mold can be done by using any ejection means suitable for one face molds. Nevertheless, this is not an optimum approach taking into account that each station must then have individual ejection and motive means. Slight improvements of the ejection methods used in conjunction with a stack mold have been already disclosed, wherein the ejection means and the opening means of the stack mold have been connected by a linking mechanism, as a means to use a single motive means to perform simultaneous opening of the stack mold and ejection of articles from all stations. This approach has two major drawbacks. First, the ejection step must follow immediately the opening of the mold, thus no window time is provided to extend the cooling of the part in the mold. Second, the common linking mechanism makes the access of a robotics arm more difficult due to the lack of space in-between the mold stations.
It is thus the main purpose of this invention to teach a novel linking mechanism among the mechanical ejection means of a plurality of molds that is independent to stack mold's linking and motive means. The linking mechanical among the ejection means may be actuated by various motive means that are either part of the mold or of the molding machine.
The patent literature teaches various ejection methods using numerous ejection means in conjunction with several actuation methods and means. For example, mechanically actuated ejection means using such devices as various knock-out rods is disclosed in U.S. Pat. No. 3,448,488 to Kiraly. Hydraulically actuated ejection means are disclosed in U.S. Pat. No. 4,752,200 to Bartschke. Using pressurized fluid, such as blown air, is another ejection method which is simpler than most mechanical methods for pushing a molded part off or out of a mold core and cavity respectively. Such a method and means is disclosed in U.S. Pat. No. 4,994,228 to Watanabe. All of these ejection methods and means are disclosed for use in conjunction with standard molds having a single molding station, i.e., one cavity plate and one core plate. While many of these methods and means may be successfully used in connection with multistation molds, such as stack molds, they represent expensive and complicated solutions since based on their design, each molding station is required to have its own independently driven and actuated ejection means.
Multistation molds such as stack molds have been in use for many decades and are also well described in the patent literature as a very productive tool for efficiently molding a large quantity of articles. A stack mold generally includes at least two separate molding stations consisting of at least one pair of cavity/core combinations. In many cases, multiple cavity/core combinations are used. During use, stack mold stations are typically all simultaneously closed, fed with molten resin, and then opened and automatically demolded using the necessary means to execute all of these steps. Because of the multiple stations, demolding or the ejection process of plastic articles from the multiple molding stations in stack molds represent a distinctive and substantially more complex issue in comparison to similar ejection steps performed in single station molds. The complexity stems from the fact that the multiple ejection mechanisms currently used in the stations of the stack molds have to be accurately coordinated among each other during the entire cycle time and especially during the opening sequence of each mold station. A variety of ejection methods and means for use with stack molds are disclosed in the prior art.
These methods and means include German Patent 2,243,823 which teaches a stack mold wherein the plastic parts are air ejected via poppets; U.S. Pat. No. 2,331,015 which teaches a stack mold having a hydraulically actuated pair of pistons mounted on a center section used to open the molds at the mold parting lines; U.S. Pat. No. 3,767,352, assigned to the assignee of the present application, which teaches a linkage mechanism contained in mold shoes that is further connected to a stripper plate. The linkage mechanism causes the stripper plate to move as the stack mold opens for ejecting molded articles. The actual motion of the stripper plate is a simple harmonic motion and happens during the opening stroke of the mold; U.S. Pat. No. 4,207,051, also assigned to the assignee of the present application, which teaches a stack mold using a rack and pinion centering means that has a stripper plate driven by an adjustable link connected to a pinion mounted on a center section of the mold.
In accordance with these previously developed stack mold ejection methods and means, several drawbacks are apparent which thus far, have not been properly addressed.
Air ejection methods have limited applications since the size, shape and/or weight of the plastic parts must be suitable, and typically are not, for this simple and inexpensive method. In U.S. Pat. Nos. 3,767,352 and 4,207,051, the ejection means is directly connected to the motive means used to open and close the mold stations of the stack mold. This is a major disadvantage since space constraints caused by connecting the ejection and linkage means to the motive means prevent robotic means from entering the molding area to pick up parts that have been already ejected during the opening of the stations. In addition, independent opening of the stack mold without ejecting a part therefrom is not possible and the actual mold opening stroke is limited. Further, by using ejection means connected to the mold opening means, a cooling step which is generally required cannot be extended after opening the molding station since the parts will be ejected immediately.
Ejection means and methods which are totally independent of the mold parting means are taught in the patent and technical literature, but those taught are not economical solutions for stack molds since each molding station in these devices is required to have its own motive and actuation means.
There exists a need, therefore, for an ejection means and method that overcomes the drawbacks of the existing ejection methods and means described in the technical literature. In this regard, the present invention teaches an ejection method and means using innovative linkage means among stripping mechanisms of each molding station and appropriate motive means to actuate the linkage means, wherein both the linkage and the motive means are independent to the linkage and motive means operating the stack mold. At least one common unit is used for driving the ejection means of all of the stations of the stack mold.